This invention relates generally to a semiconductor device and manufacturing method in which LSI bare-chip components are mounted on a printed circuit board with flip-chip bonding technology, and more specifically, to a semiconductor device having improved reliability and productivity.
Electronic equipment manufacturers have manufactured electronic equipment by purchasing packaged LSIs (Large Scale Integrated Circuits) from semiconductor device suppliers and mounting them on printed circuit boards (PCBs). In recent years, however, there has been a growing demand for smaller-sized electronic equipment, and electronic equipment manufacturers have met this demand by purchasing LSI bare chips (bare LSI components which are formed on a silicon substrate in a state where only electrodes or bonding pads are exposed from the final passivation film) from semiconductor suppliers and mounting them on a printed circuit board.
There are several methods of mounting bare chips components (or flip-chip components). One method is to mount bare chips on a printed circuit board by bonding (die bonding) the bare chips on the board while positioning the electrodes of the bare chips in the same direction as the electrodes of the board, and electrically connecting the electrodes of the chips and the board with wire bonding. Another method is to mount bare chips on a printed circuit board by facing the electrodes of the bare chips to the electrodes of the board and directly bonding the electrodes of the bare chips to the electrodes of the board.
The latter method is called face-down mounting, or flip-chip bonding, which has an advantage over the former method (face-up bonding) in that bare chips can be mounted in high density while having a disadvantage of low reliability and productivity. In order to meet the recent demand for smaller-sized electronic equipment, it is necessary to establish a method of mounting bare chips in high density and with higher reliability and productivity to solve the disadvantages of the flip-chip bonding process.
FIG. 39 illustrates a conventional structure of a semiconductor device in which LSI bare-chip components are mounted on a printed circuit board, or a substrate, with the flip-chip bonding technology. The structure shown in FIG. 39 was disclosed in the Proceeding of 1993 International Conference Multichip Module (Apr. 14-16, 1993, Radison Hotel, Denver).
As shown in the figure, a conventional type of a semiconductor device has a structure in which LSI bare chips are mounted with the flip-chip bonding technology, and has a raised electrode 4, called a bump, provided on a chip pad 3 of a bare chip component (or a bare chip) 1 and coated with electrically conductive paste 5 or solder (hereinafter generically referred to as electrically conductive paste for the sake of simplicity). The bare chip 1 is bonded to a printed circuit board or a substrate 2 with adhesive 7 by flipping over the bare chip 1 on the printed circuit board 2 to attach the bump 4 coated with the conductive paste 5 to the substrate pad 6 of the substrate 2.
Typically the bare chip 1 comprises a silicon substrate about 10 mm.times.10 mm in size and about 0.4 mm in thickness, and has 200 to 300 chip pads 3 comprising aluminum or other metal layers each about 100 .mu.m.times.100 .mu.m in size and a few .mu.m in in height. The gaps between the surface of the bare chip 1 and the surface of the substrate 2 is approximately 80 .mu.m, that is the height of the bump 4 is approximately 50 .mu.m and the height of the substrate pad 6 approximately is 25 .mu.m, respectively.
The semiconductor device of this type has been manufactured in the manufacturing process shown in FIG.40.
First comes the bare-chip bump forming process in which the tip of a gold wire protruding from the tip of a capillary is melted by electric discharge into a ball, which is in turn bonded to the chip pad 3 of the bare chip 1 with ultrasonic vibration (wire bonding method), and the gold wire is then cut at the neck of the ball to form a spherical bump 4 approximately 60 .mu.m in size on the chip pad 3 of the bare chip 1.
Next comes the bump leveling process in which the bare chip 1 is gently forced onto a flat glass substrate so as to eliminate variations in the height of the cut portions of the bumps 4, making the cut portions of the bumps 4 uniform in height.
Then, the conductive paste transfer process follows. Another glass substrate is prepared on which an approximately 15 .mu.m thick film of electrically conductive paste (adhesive containing metal fillers) is applied. The bare chip 1 is forced on the glass substrate to transfer the conductive paste 5 on the top surface of the bump 4. Then, the conductive paste 5 adhering to the bump 4 is half-hardened by heating the bare chip 1 for a certain period.
In the subsequent mounting process, the bare-chip component 1 is aligned with the substrate 2 on which the adhesive has been applied, and heated for a certain period while holding it in position by exerting a certain load to harden the adhesive 7 between the bare chip 1 and the substrate 2. After that, the semiconductor device is completed by perfectly hardening the conductive paste 5.
As described above in the conventional structure, a bare chip 1 is bonded to a substrate 2 with adhesive 7, while bringing a bump 4 formed on the chip pad 3 of the bare chip 1 into close contact with the substrate pad 6 of the substrate 2, by employing an electrically conductive paste 5 to ensure reliability in electrical connection between the bump 4 and the substrate pad 6.
With the prior-art structure, however, productivity in manufacturing semiconductor devices has not necessarily been good.